Thermo hydraulics performance of turbulent flow heat transfer through square ducts with inserts

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND
TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 11, November (2014), pp. 59-65
© IAEME: www.iaeme.com/IJMET.asp
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IJMET
© I A E M E
THERMO HYDRAULICS PERFORMANCE OF
TURBULENT FLOW HEAT TRANSFER THROUGH
SQUARE DUCTS WITH INSERTS
M. Udaya Kumar
Associate Professor, Indur Institute of Engineering and Technology, Siddipet, MEDAK (District),
Telangana - 502 277, India
M. Manzoor Hussian
Professor, JNTU Hyderabad, Kukatpally, Hyderabad, Telangana - 500085
Md. Yousuf Ali
Nawab Shah Alamkhan College of Engineering and Technology,
No.16-4-1, New Malakpet, Near Railway Station, Malakpet, Hyderabad, Telangana 500024
ABSTRACT
This paper describes the experimental study of square ducts with inserts. Experiments are
conducted for air with uniform heat flux condition. The top wall surface is made rough with metal
ribs of square section. The roughened wall is uniformly heated and other walls are insulated. The
heat transfer coefficient enhances square channel at injection of different inserts. The performance of
the geometry under investigation has been evaluated .The heat transfer coefficient of air is increase
by 46% than plane square ducts with inserts. The heat transfer and pressure drop measurements have
been taken in separate sections .The flow friction and thermal characteristics are governed by duct
aspect ratio, twist ratio, length, Reynolds number and prandtl number. The heat transfer and pressure
drop characteristics of turbulent flow of air (10.000Re100,000) through square ducts with inserts
on all surfaces of the ducts have been studied experimentally.
INTRODUCTION
The enhancement of single phase heat transfer inside a duct is often achieved by forming
some swirling or secondary flows. It is usually accompanied with high turbulence intensity, which

2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME
promotes the mixing of different parts of fluids, hence enhances the heat transfer. The swirling and
secondary flow can be initiated by several ways, among which the insert of twisted tape or wire coil,
the injecting or imparting a tangential flow, the use of integral tube with helical fins, spiral fluted
duct and the twisted duct are often encountered. Most of these methods have been intensively
studied. For example, the heat transfer and fluid flow behavior of the twisted tape have been studied
and compared comprehensively by Manglik and Bergles [1],[2]. Yampolsky [3] and Obot et al. [4]
tested the fluid flow and heat transfer characteristics of the spirally fluted tubes. The imparting of a
tangential flow was investigated by Gau and Chen[ 5]. Relatively speaking the study on the twisted
duct is quite limited. The purpose of this paper is to investigate the turbulent heat transfer and fluid
flow in twisted uniform cross section, converging and diverging square ducts. As a technique for
enhancing heat transfer, twisted duct has a unique feature in that it may induce swirling secondary
flow and increase fluid contacted surface in some extent within a given axial length while still
keeping smooth surfaces. Apart from the enhancement consideration, the present study was also
motivated by the need in understanding the heat transfer mechanism of gas turbine blade cooling
process. The cooling channel of a gas turbine is often modeled by a duct with a square cross section
[6]. Periodic ribs are frequently employed to enhance the heat transfer process in various cooling
passages such as turbine blades, guide vanes and combustor walls. Heat transfer augmentation inside
cooling channels is achieved by using repeated ribs as turbulence promoters. The periodic ribs break
the laminar sub-layer and create local wall turbulence due to flow separation and reattachment
between them ribs, greatly enhancing the heat transfer. Several researchers have studied the heat
transfer characteristics in straight channels with various shaped ribs using air as coolant flow.
Chandra et al. [7], Han et al. [8], Lanjewar et al. [9], Srinath et al. [10], Salameh and Sunden [11],
Tanda [12] and Wang and Sunden [13] studied experimentally the effect of ribs configuration and
angled ribs on heat transfer and friction. While by numerical predictions of the flow and heat transfer
in rib-roughened passages have been conducted previously by several investigators: Kashmiri et al.
[8] investigated the rib pitch effect on heat transfer. Insertion of twisted tapes in circular tubes
provides a simple passive technique for enhancing the convective heat transfer coefficient on the
tube side of a heat exchanger. Twisted tape inserts are used to achieve compact heat exchangers, as
well as, to prevent hot spots in high heat flux transfer situations encountered with gas flows. A duct
of square cross-section provides higher surface to volume ratio than a circular tube. Further, if a
square duct is inserted with a twisted tape, whose width equals the side of the duct, the flow becomes
periodically fully developed with the distance of periodicity equals to 900 rotation of the tape.
Similarly, the heat transfer also attains periodically fully developed state. Thus, both the flow and
heat transfer are under continuous state of periodic development. Therefore, compared to a circular
tube with a twisted tape insert, a higher thermal hydraulic performance can be expected from a
square duct with a twisted tape insert. Mano et al. [14] and Bhadsavle [15] have presented
experimentally determined correlations of friction factors for flow in a square duct with twisted tape
insert. On comparison, however, it is observed that the correlations are at considerable variance with
each other. In general, turbulent heat transfer in the duct is dominated by the transport of heat by
turbulence. Therefore, in order to understand the mechanism of turbulent heat transfer in the
complex through a square duct, the characteristics of the temperature flow field, such as temperature
fluctuation intensity and turbulent heat fluxes, should be examined detail. Square ducts are widely
used in heat transfer devices. For instance in compact heat exchangers, gas turbine cooling systems,
cooling channels in combustion chambers and nuclear reactors. The forced turbulent heat convection
in a square duct is one of the fundamental problems in the thermal science and fluid mechanics. The
problem of turbulent heat and fluid flows in a straight square duct is fundamental in thermal science
and fluid mechanics. The turbulence in the straight square duct has a remarkable change in Flow
structure due to existence of the so called Prandtls second kind secondary flows. Enhancement leads
to reduce size and cost of heat exchanger. An increase in heat transfer coefficient generally leads to
60

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME
additional advantage of reducing temperature driving force. Whenever inserts are used for the heat
transfer enhancement, along with the increase in the heat transfer rate, the pressure drop also
increases. This increase in pressure drop results in rise in pumping cost. Therefore, any augmentation
device should optimize between the benefits due to the increased heat transfer coefficient and the
higher cost involved because of the increased frictional losses.
61
EXPERIMENTAL SET UP
Schematic diagram of the experimental set-up is shown in Fig1. Experiments were carried
out in a double-pipe heat exchanger. It has square duct (Aluminum duct, D = 25 mm, L = 4000 mm,
AR =1.0, thickness () = 4 mm) .It consists of calming section, test section, U Tube Manometers, air
was flowing through the inner square duct at very high flow rate in counter-flow, through the annular
channel formed between inner square duct and outer surface to attain nearly uniform wall
temperature conditions. The temperature of hot air is maintained at desired set point with the
deviation of plus or minus 10C by controlled heating. All the RTD PT 100 type (Teflon coated)
temperature sensors used to measure inlet, outlet temperatures of hot air. The wall temperature is
obtained by taking average of all thermocouples installed in axial location along the wall of the
duct. A groove of 1 mm depth is made on outer wall of the duct, temperature measuring probe
inserted in to the groove and adhesive was applied around the groove to fix up the thermocouples to
the wall. This is particularly essential to get inner wall temperature. The test section and connected
piping were wound with asbestos insulation of approximately 30 mm thickness to minimize heat loss
to the surrounding area. The flanges are used to connect calming section to test section and mixing
section at the other end. Mixing section (stainless steel, square duct, 500 mm L and 25.2 mm De),
whose 100mm length filled with wire mesh, Teflon gaskets of 8 mm thickness placed in between
flanges to prevent heat conduction loss to calming section and mixing section. The pressure drop
across the test section was measured by using vertical inclined U-tube manometer containing water.
Heat Transfer calculations
Ts = (T2+T3+T4+T5+T6+T7+T8+T9)/8
Tb= (T1+T10)/2
Mean temperature, Te = ( Ts+Tb ) / 2
Reynolds Number,
Re = De v/μ
De = 4A /p
Nusselt Number:
Nu = hiDe /k
Experimental Nusselt Number (Nu):
Heat transfer coefficient, h = q/(Ts-Tb)
Heat flux, q =Q/A
(2)Theoretical Nusselt Number (Nuth): 0.024 Re
0.8 Pr 0.4
f =[(P /L)De )]/[(U2 /2)]

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME
62
SCHEMATIC DIAGRAM
Nomenclature
A convective heat transfer area (DL), (m²)
Qd air discharge through test section (m³/sec)
Dh hydraulic diameter (4A/P), (m)
F friction factor
fi friction factor obtained using inserts
h experimental convective heat transfer coefficient, (W/m²K)

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME
63
hair equivalent height of air column, (m)
k thermal conductivity, (W/mK)
L length of test section, (m)
m mass flow rate of air, (Kg/sec)
Nui Nusselt number (experimental) with inserts, (hDh/k)
Nu Nusselt number (experimental) for plain square duct
Nuthe Nusselt number for plain square duct(theoretical)
Pr Prandtl number
P wetted perimeter, (m)
P pressure drop across the test section, (Pa)
Re Reynolds number,
T1, T10 - air temperature at inlet and outlet, (°C)
T2, T3
T4, T5 - duct wall temperatures, (°C)
T6, T7
T8, T9
T s average Surface temperature of the working fluid, (°C)
T bulk temperature, (°C)
Te means temperature, (°C)
V velocity of flow (m/sec)
U air velocity through test section, (m/sec)
Greek symbols
Kinematic viscosity of air, (m²/sec)
μ dynamic viscosity, (kg/m s)
Over all enhancement ratio
a density of air (Kg/m³)
v velocity (m/sec)
Thermo hydraulic performance
For a particular Reynolds number, the thermo hydraulic performance of an insert is said to be
good if the heat transfer coefficient increases significantly with a minimum increase in friction
factor. Thermo hydraulic performance estimation is generally used to compare the performance of
different inserts such as twisted tape, wire coil, etc., under a particular fluid flow condition.
Overall enhancement ratio
The overall enhancement ratio is defined as the ratio of the heat transfer enhancement ratio to
the friction factor ratio .This parameter is also used to compare different passive techniques and
enables a comparison of two different methods for the same pressure drop. The friction factor is a
measure of head loss or pumping power.
Nusselt number
The Nusselt number is a measure of the convective heat transfer occurring at the surface and
is defined as hd/k, where h is the convective heat transfer coefficient, d is the diameter of the tube
and k is the thermal conductivity.

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME
64
Prandtl number
The Prandtl number is defined as the ratio of the molecular diffusivity of momentum to the
molecular diffusivity of heat.
Aspect ratio
It is ratio of duct width to duct height. This factor also plays very crucial role in investigating
thermo hydraulic performance.
Pitch
Pitch is defined as the distance between two points that are on the same plane, measured
parallel to the axis of a twisted tape.
CONCLUSIONS
1. The friction factor and Nusselt number in square duct is higher than that for circular tube
under a same operating conditions. This happen due to fact that a duct of square cross section
provides higher surface area to volume ratio than that of circular tube when both were fitted
with different inserts.
2. Experimental results of plain square duct were compared and found in good agreement with
well known correlation of friction factor.
3. The friction factor decreases with increasing Reynolds number and increases with decreasing
twist ratio.
4. The increase in friction factor and heat transfer can be explained by the generation of swirling
flow as result of secondary flows of fluid.
5. In the present study experimental set up was validated for friction factor and heat transfer
results. Isothermal friction factor and Nusselt number variation with Reynolds number for a
square duct fitted with different inserts are also investigated.
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